The complex Fourier series of a periodic function $f(x)$ of period $2l$, defined on the interval $(-l,l)$ is $$f(x)= \sum_{n=-\infty}^{+\infty} c_n e^{\pi i nx/l}$$with the coefficients $c_n$ given by the formula\begin{equation*}c_n=\frac{1}{2l} \int_{-l}^{l} f(x) e^{-\pi i nx/l} dx\end{equation*}